Conventionally, during nighttime, especially prior to bedtime, it is known that a stimulation of an encephalon is reduced by configuring a color temperature of a lighting in a living space to a lower color temperature (reddish color light such as a light bulb) when compared to using a white color light and a daylight color light and that a smooth sleep is facilitated by. A technology for varying a color of light according to time or a person's biorhythm has been proposed (for example, see patent reference 1).
With respect to the above, a novel idea regarding a relation between a wavelength of a light and a biorhythm and a sleep is reported in a paper of non-patent reference 1.
In accordance with the non-patent reference 1, a wavelength characteristic of a suppression of the melatonin secretion by a reception of light in the nighttime is disclosed. Melatonin is a hormone secreted from a pineal gland in the encephalon and is secreted from prior to sleep to the entire sleep time (from 10 P.M. to late night although dependent upon individual or biorhythm). It is known to stimulate a body temperature drop or a sleep.
The melatonin is known as what its secretion is suppressed by the reception of the light in the nighttime, and an action spectrum representing the wavelength characteristic as shown in FIG. 1 is disclosed in the paper. As shown in FIG. 1, the wavelength where a sensitivity of the suppression of the melatonin secretion is at its peak is 464 nm, and the suppression of the melatonin secretion is prevented by maximally blocking a neighboring wavelength component close to the wavelength of 464 nm.
In addition, as shown in spectral distributions of various light sources in FIG. 2, the wavelength component exists in a light source having a low color temperature, an incandescent lamp or an incandescent color fluorescent lamp generally used in an indoor lighting space although it is small in amount when compared to a white color or an daylight color fluorescent lamp. Therefore, this induces the suppression of the melatonin secretion (see non-patent reference 2).
Based on the above-described idea, a technology regarding a lighting apparatus and a light source for use with the same has been developed for embodying security of visual information without hindering sleep due to the suppression of the melatonin secretion by the reception of light in the nighttime (by way of an example, see patent reference 2). In accordance with the patent reference 2, blocking or attenuating a light of a wavelength of about 410˜505 nm which has a high suppression effect of the melatonin secretion in the nighttime has been proposed. Moreover, by adding or passing through a light of a wavelength about 400 nm, an effective way of preventing the suppression of the melatonin secretion has been proposed while maintaining a color of the light desirable.
In addition, it is known that irradiating an intense light in a broad daylight, especially in the morning, has an effect of adjusting a biorhythm. However, based on the above-described idea, by actively using a light having a center wavelength of 464 nm to the contrary, a method for effectively adjusting the biorhythm has been proposed (see patent reference 3).
As described above, in order to create a light which does not suppress the melatonin secretion, a method for blocking or attenuating a wavelength which suppresses the melatonin secretion by using an optical filter can be considered other than using a light that does not generate a wavelength which suppresses the secretion of melatonin.
[Patent reference 1] Japanese patent publication no. 2000-252084
[Patent reference 2] Japanese patent application no. 2004-041545
[Patent reference 3] Japanese patent application no. 2004-128465
[Non-patent reference 1] G. C. Brainard et al. (2001): ‘Action Spectrum for Melatonin Regulation in Humans: Evidence for a Novel Circadian Photoreceptor’, The Journal of Neuroscience, Aug. 15, 2001, 21(16), pp 6405-6412
[Non-patent reference 2] Sato M, Noguchi H, Morita T (2004): The Effects of Light with Different Spectral Distribution During the Nighttime on Melatonin Secretion and Psychological Factors in Humans, CIE Expert Symposium on Light and Health abstract, 87-88(Sep. 30-Oct. 2, 2004, pp 87-88)
As described above, when viewed from an aspect of melatonin stimulating the body temperature drop or the sleep in the nighttime, the reception of the light containing the wavelength range, prior to sleep in the nighttime, should be avoided for a fine quality of sleep, and should also be coped with in a level of a lighting apparatus or a light source.